Striated muscle cells consist of three sets of filaments: (1) thick, myosin containing filaments, which interdigitate with (2) thin, actin-containing filaments and (3) titin filaments, which center the thick filaments axially in the sarcomere and produce the resting tension. The resting tension is different in fibers taken from different muscles: the tension produced by individual rabbit soleus muscle fibers is about five times smaller than that produced by rabbit psoas muscle fibers. sensitive quantitative electrophoretic measurements show that the concentration of the major myofibrillar proteins is the same in both types of fibers. Therefore the different tension levels in the soleus and the psoas fibers are due to a qualitative difference in the elastic properties of the titin molecules in the different muscle fibers. A widely accepted model of muscle contraction postulates that force is generated by "rocking" the head part of the myosin molecule after it attaches to the actin filament. This model is supported by X-ray reflections from fibers put into the rigor state, cross-linked with EDC, and then strained by 1% in the presence and absence of ATP analogues. The 1,1 reflection decreases by about 10% at a rate that depends on the particular ATP analogue. The results suggest that an EDC tethered crossbridge becomes more orthogonal to the thin filament axis when strained. The pattern of dissociation of thick filaments in rigor muscle fibers fixed for electronmicroscopy under low salt, high pH conditions shows that crossbridges from a given myosin subfilament can attach to at least three of the adjacent thin filaments.